Vehicles have been developed to perform idle-stop when idle-stop conditions are met and automatically re-start the engine when re-start conditions are met. A number of methods may be used to enable a smooth launch control of the automatic transmission when re-start conditions are met. In one example, the vehicle may be started in a “Drive” gear and further coordination of an engine rotation speed and an engagement state of one or more forward clutches may enable vehicle launch with minimal “lurching”.
One such example of a vehicle re-start and launch method is illustrated by Katou et al. in EP 1,348,895A2. Herein, a method is described for a smooth launch control of an automatic transmission vehicle using a hydraulic system and by detecting an engagement pressure of the forward clutch. Specifically, if the engagement pressure is below a predetermined value, an engine torque reduction is commanded by an electronic controller 12 until the desired forward clutch engagement pressure is reached. The method also enables an adaptive control of the timing of an initial engine combustion based on a time to the predetermined forward clutch engagement pressure.
However, the inventors herein have recognized potential issues with such an approach. As one example, modulation of the forward clutch during a majority of automatic re-starts may lead to excessive wear of the forward clutch. As another example, if the system is tuned for level grades, insufficient driveline torque may be generated when the vehicle is launched on a grade.
Thus in one example, some of the above issues may be addressed by a method for controlling a system including an engine. The method may include operating in a first mode during a first vehicle grade and during automatic re-starting from an engine stopped condition. The first mode may include starting the engine in a driving gear, and adjusting engine torque responsive to launch behavior. The method may further include operating in a second mode during a second vehicle grade and during automatic re-starting from an engine stopped condition. The second mode may include starting the engine in the driving gear, and adjusting one or more of a transmission forward clutch and wheel brakes responsive to launch behavior.
In this way, it is possible to reduce clutch wear while also addressing the issue of consistent launch performance on various grades. For example, in one example, on lower grades, fuel economy concerns may drive the selection of engine torque modulation as the driveline torque modulator. In such a case, engine torque may be adjusted responsive to vehicle launch behavior, such as vehicle speed, engine speed, and clutch pressure, etc. By using at least engine torque modulation for automatic re-starting on lower grades, forward clutch degradation as well as brake caliper wear may be reduced. In another example, on steeper grades, coordinated adjustment of the forward clutch and/or wheel brakes responsive to vehicle launch behavior, including vehicle speed, engine speed, clutch pressure, and other control parameters, may be used to modulate driveline torque during the automatic re-start. Thus, by considering longitudinal vehicle grade, a smoother launch may be achieved more consistently, across a range of grades, while reducing wear of the forward clutch and brake caliper. Further, fuel economy benefits of the vehicle may also be achieved by selectively using engine torque modulation under certain conditions, and forward clutch modulation and/or wheel brake modulation under other conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.